<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>

    </style>
</head>
<body>
<script>

    function fn(x, y, count) {
        if (count !== 5) {      //  n
            let total = x + y
            x = y
            y = total
            console.log(total)
            return fn(x, y, ++count)
        }
        return x + y
    }

    console.log(fn(1,1,0))

    // 第 n + 3 项是 21
</script>
</body>
</html>